[Performance][VB.NET]A Thorough Study of Empty String Checks in .NET

.NET下的空字串判斷整體來說大概可分為下列幾種方法：

1. 用 is Nothing 判斷。e.x. If str Is Nothing
2. 用 = Nothing 判斷(類別中只有字串可以用 = Nothing 來判斷)。e.x. If str = Nothing
3. 用 = "" 判斷。e.x. If str = ""
4. 用 = String.Empty 判斷。e.x. If str = String.Empty
5. 用 Is String.Empty 判斷。e.x. If str Is String.Empty
6. 用 String.IsNullOrEmpty(str) 判斷。e.x. If String.IsNullOrEmpty(str)
7. 用 String.Length = 0 判斷。e.x. If str.Length = 0

上述所列出的空字串判斷方法，其所跑出的效果不盡相同，為了看出其差異性，這邊撰寫了簡單的測試程式如下：

Sub Main()
    Dim testStrings() As String = New String() {Nothing, "", String.Empty}
    For Each str As String In testStrings
        TestString(str)
        Console.WriteLine("=======================")
    Next
End Sub

Private Sub TestString(ByVal str As String)
    If str Is Nothing Then Console.WriteLine("str Is Nothing")
    If str = Nothing Then Console.WriteLine("str = Nothing")
    If str = "" Then Console.WriteLine("str = """"")
    If str = String.Empty Then Console.WriteLine("str = String.Empty")
    If str Is String.Empty Then Console.WriteLine("str Is String.Empty")
    If String.IsNullOrEmpty(str) Then Console.WriteLine("String.IsNullOrEmpty(str)")

    Try
        If str.Length = 0 Then Console.WriteLine("str.Length = 0")
    Catch ex As Exception

    End Try
End Sub

執行結果如下圖所示：

由上圖可知， "" 與 String.Empty 結果是一樣的。比較有趣的是，當字串值是"“與String.Empty時， str is Nothing 是不會成立的，但是 str = Nothing 卻會成立。而當字串參考是 Nothing 時，str is string.Empty 不成立、 str = string.Empty 成立。這是因為當使用=運算子判斷時，Nothing 、 "” 與 String.Empty 是被視為一樣的，才會造成此有趣的現象。

接著我們來比較一下空字串判斷的執行速度，同樣的我們須撰寫如下測試程式：

Sub Main()
      Dim testStrings() As String = New String() {Nothing, "", String.Empty}
      For Each str As String In testStrings
          TestStringSpeed(str, 10000000)
          Console.WriteLine("=======================")
      Next
  End Sub

  Private Sub TestStringSpeed(ByVal str As String, ByVal count As Integer)
      Dim n As Integer = count
      Dim sw As New Stopwatch
      Console.WriteLine("str Is Nothing")
      sw.Start()
      For i As Integer = 1 To n
          If str Is Nothing Then
          End If
      Next
      Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

      Console.WriteLine("str = Nothing")
      sw.Reset()
      sw.Start()
      For i As Integer = 1 To n
          If str = Nothing Then
          End If
      Next
      Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

      Console.WriteLine("str = """"")
      sw.Reset()
      sw.Start()
      For i As Integer = 1 To n
          If str = "" Then
          End If
      Next
      Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

      Console.WriteLine("str = String.Empty")
      sw.Reset()
      sw.Start()
      For i As Integer = 1 To n
          If str = String.Empty Then
          End If
      Next
      Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

      Console.WriteLine("str Is String.Empty")
      sw.Reset()
      sw.Start()
      For i As Integer = 1 To n
          If str Is String.Empty Then
          End If
      Next
      Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

      Console.WriteLine("String.IsNullOrEmpty(str)")
      sw.Reset()
      sw.Start()
      For i As Integer = 1 To n
          If String.IsNullOrEmpty(str) Then
          End If
      Next
      Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

      Try
          Console.WriteLine("str.Length = 0")
          sw.Reset()
          sw.Start()
          For i As Integer = 1 To n
              If str.Length = 0 Then

              End If
          Next
          Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)
      Catch ex As Exception

      End Try
  End Sub

執行結果如下：

從上圖可看出幾個現象

1. 用String.IsNullOrEmpty同時判斷Nothing與空字串會比只判斷是否為 Nothing 或為空還慢
2. 當字串不為 Nothing 時要判斷字串是否為空的話，用Is String.Empty去判斷最快(文件上通常是說String.Length = 0最快)

接著再看個實驗來看當同時判斷Nothing與空字串的情況，測試程式如下：

Sub Main()
     Dim testStrings() As String = New String() {Nothing, "", String.Empty}
     For Each str As String In testStrings
         'TestString(str)
         TestStringSpeed(str, 10000000)
         Console.WriteLine("=======================")
     Next
 End Sub

 Private Sub TestStringSpeed(ByVal str As String, ByVal count As Integer)
     Dim n As Integer = count
     Dim sw As New Stopwatch
     Console.WriteLine("str Is Nothing & Is String.Empty")
     sw.Start()
     For i As Integer = 1 To n
         If str Is Nothing Then
         Else
             If str Is String.Empty Then
             End If
         End If

     Next
     Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

     Console.WriteLine("str Is Nothing & Is String.Empty")
     sw.Reset()
     sw.Start()
     For i As Integer = 1 To n
         If str Is Nothing Then
         Else
             If str.Length = 0 Then
             End If
         End If
     Next
     Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

     If str = Nothing Then Console.WriteLine("str = Nothing")
     sw.Reset()
     sw.Start()
     For i As Integer = 1 To n
         If str = Nothing Then
         End If
     Next
     Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

     Console.WriteLine("str = """"")
     sw.Reset()
     sw.Start()
     For i As Integer = 1 To n
         If str = "" Then
         End If
     Next
     Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

     Console.WriteLine("str = String.Empty")
     sw.Reset()
     sw.Start()
     For i As Integer = 1 To n
         If str = String.Empty Then
         End If
     Next
     Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

     Console.WriteLine("String.IsNullOrEmpty(str)")
     sw.Reset()
     sw.Start()
     For i As Integer = 1 To n
         If String.IsNullOrEmpty(str) Then
         End If
     Next
     Console.WriteLine("花費時間: " & sw.ElapsedMilliseconds)

 End Sub

執行結果如下：

可以看出 String.IsNullOrEmpty 函式的效能與自行混用 str Is Nothing 與 Is String.Empty 判斷方法來達到相同目的的效能差不多。

經過以上三個實驗，對於字串為空的判斷相信已有相當的了解，若能清楚分辨其功用與使用的時機，多少都能增進程式的效能。像是使用上若只需判斷Nothing或為空的話，應避免誤用會同時判斷的方法(如下例)，就可以避免不必要的OverHead。